In the manufacture of a wide variety of molded articles it is desired to change one or more elements of a mold from time to time for production changes and/or maintenance purposes. Conventionally, mold elements such as mold inserts, the mold cavity and/or the mold core plates have been retained in place on the mold machine via manually activated retaining and releasing means such as bolts, etc. While such means provide good mechanical strength and can be inexpensive to employ, mold elements retained by them are generally not easily or quickly removed from the mold machine.
The use of conventional fasteners is especially problematic in the molding of articles which require frequent and/or prompt removal and replacement of one or mold elements. For example, in molding automotive bumpers, an element in a mold may need to be changed during a production run to allow different configurations of the bumper to be molded as necessary. A standard bumper may be molded for the majority of cars but an optional lighting package may require that the bumpers molded for some cars include receptacles for fog lamps, or the like. With conventional means of retaining the mold elements, changing the mold elements to allow production of the bumper for optional lighting packages can be onerous and/or time consuming.
The mold element that has to be changed rapidly can be the entire mold, or just one mold half. Reference is made in this regard to the U.S. Pat. No. 5,486,101 to Guyon that shows a mold clamping mechanism including a transverse cylinder and a wedge that interacts with a peg attached to the mold half to be retained. The mold half is thus removable connected to an injection molding machine platen. Unfortunately, the design of Guyon requires a lot of space due to the fact that the cylinder is in a transverse position in order to move the wedge laterally with respect to the peg. Also, if many changes of the mold half are required, the working surfaces of the wedge and of the peg can be worn out due to the high friction they are exposed to in use. This can cause the peg to become loosely connected to the machine platen after a certain time. Therefore the design concept of Guyon is not applicable to applications which require many changes and which require accurate positioning, such as the retention of the stamper plate in a CD mold. The stamper plate has to be replaced very fast, accurately and typically tens of times during a 24 hours manufacturing cycle.
Information carrier articles such as CDs (compact discs) or DVDs (digital versatile discs) are typically molded in a three part mold, comprising a stationary mold plate, a movable mold plate and a stamper plate. The stamper plate is a removable mold element which is maintained in the mold cavity formed between the stationary and the movable mold plates. The stamper is a plate-like disc element with upstanding bosses responsible for forming the encoded digital information carrier pits in the molded final article. Each time a different information title is to be molded, the stamper is changed appropriately.
The technical and patent literature disclose several ways to retain the stamper plate in a CD mold. The first way is to hold directly the stamper plate using vacuum or magnetic clamping means as shown for example in the U.S. Pat. No. 5,552,098 to Kudo and more recently in the U.S. Pat. No. 5,612,062 to Takahashi. The second way is to use a central holder attached to the stamper plate that is retained using either mechanical means or vacuum means. U.S. Pat. No. 5,116,210 of Watanabe et al. shows the mechanical approach and U.S. Pat. No. 5,297,951 of Asai shows the vacuum approach. A third way is to use a combination of the direct and the central holding means. The direct means can be a ring pressing the outer diameter of the stamper plate which is employed with a central holding means. This combined approach is disclosed amongst other in the U.S. Pat. No. 4,917,833 of Cools and U.S. Pat. No. 5,297,951 of Asai.
While under ideal circumstances the run size of each information title is sufficiently large to mitigate the frequency with which the stamper must be changed (to, for example, once a day), often this is not the case and it is desired to change a stamper several times a day. In most instances the stamper plate is changed using manual means to release the central holder, as shown in the U.S. Pat. No. 5,116,210 of Watanabe, U.S. Pat. No. 5,466,145 of Takahashi and more recently in the U.S. Pat. No. 5,607,705 of Asai. In other instances, automatic means are used to release the stamper plate from the outer ring as shown in U.S. Pat. No. 4,917,833 of Cools.
In addition to the inconvenience of frequent stamper changes, such systems are very slow and thus suffer from the problem that the resin used to mold the information carriers undergoes an undesired crystallisation when it remains in the hot runner of the molding machine for more than a short period of time. Thus, as the change of the stamper in systems such as that shown in the above-mentioned Watanabe, Takahashi, and Asai references can take a significant time to accomplish, the crystallised resin must be purged from the mold machine after each stamper change. This results in a significant amount of waste material and additional manufacturing down time. In addition, a dedicated person is required to select the new stamper plate and to handle the removal and storage of the previous stamper plate and the loading of the new stamper plate.
Previous attempts have been made to address this problem. In this regard, reference is made to Swiss Patent CH 663 929 to Muller et al., the U.S. Pat. No. 5,374,177 to Hehl, PCT publication WO/94/19166 to De Graaf and the Japanese Patent 62-286717 to Shuji et al. While both Muller and De Graaf show known spigot means, robotics systems and stamper storage magazines used during the stamper change process, all of these patents fail to teach enabling designs of the mold itself and, more importantly, fail to teach the actual means used to retain the stamper plate.
Attempts have been made in the past to provide systems for changing a mold element such as a stamper plate in an efficient manner by improving the retaining and releasing means of the stamper plate. For example, U.S. Pat. No. 4,789,320 to Sasamura et al. assigned to the assignee of the present invention and U.S. Pat. No. 4,917,833 to Cools teach an all mechanical stamper plate retention means employing a central spigot attached to the stamper plate and an outer ring independent to the stamper plate that is used to hold the periphery of the stamper disc. In both embodiments, the outer ring has to be firstly disengaged prior to releasing the stamper plate and the spigot from the mold in order to change it.
More specifically, Cools shows a spring based stamper releasing mechanism comprising in the mold half a movable thrust plate connected to a ball bushing. This ball bushing further comprises several circumferential bores wherein balls are located to engage changer sleeve through the annular groove adapted to catch the balls. An outer bushing also has groove adapted to release the balls when they come in alignment. To retain the stamper plate and its support, thrust pins attached to the thrust plate are pushed backward by a guard ring. The guard ring remains attached to the mold plate in the molding position. To replace the stamper plate, the guard ring is rotated by two pneumatic motors and is disconnected from the mold plate and attached to the mold plate. In this way stamper plate can be replaced due to the ejection force upon the sleeve. This design is not suitable for most CD applications as it requires a mechanical guard ring to hold the exterior of the stamper and to further activate the spring based retaining/releasing elements.
Other versions of a retaining mechanism of a stamper plate central holder using threaded stamper holders actuated by gears are shown in Watanabe, Takahashi and Asai. U.S. Pat. No. 5,466,145 to Takahashi shows three related methods of maintaining a stamper in a mold. FIG. 3 of this reference shows a prior art stamper retaining mechanism wherein the stamper is mounted to a centre bush which includes a threaded portion distal the stamper. These threads are engaged by a face gear which is rotated within the mold plate by a geared drive to draw the bush into a bore in the mold plate and toward the face gear, urging the stamper into contact with the mold plate. FIG. 4 of this reference shows a similar prior art system wherein the centre bush includes a locking ring distal the stamper, instead of the threaded portion. In this embodiment, a pair of horizontal control rods extend through the mold plate and into each side of the bore into which the bush is received. The portions of the control in the bore include cam surfaces which engage the locking ring of the bush to urge the stamper into engagement with the mold plate when the control rods are rotated. FIGS. 1 and 2 of this reference show a system wherein the centre bush includes a threaded portion, similar to the embodiment shown in FIG. 3, which is engaged by a pair of control rods, similar to those of FIG. 4, but which include toothed portions to engage the threaded portion, rather than cam surfaces.
However, the mechanisms shown in the Takahashi reference also suffer from disadvantages. Specifically, as stated by Takahashi, the face-gear mechanism of the embodiment of FIG. 3 occupies a significant volume within the mold plate, thus interfering with the routing and sizing of mold cooling systems within the mold plate. Also, the embodiment of FIG. 4 suffers from the disadvantage that the extent to which the stamper may be lifted above the mold plate is limited by the diameter of the control rods and their cam surfaces. Also, the embodiment of FIGS. 1 and 2 is somewhat delicate, requiring proper engagement of the threaded portion of the centre bush with the toothed control rods. Finally, each of these embodiments is intended to be operated by a human operator who is responsible for changing the stamper and do not lend themselves well to use with automated and/or robotic systems for changing the stampers.
U.S. Pat. No. 5,401,158 to Kubota et al. shows a different concept to exchange a first stamper unit with another. Essentially, each stamper is mounted to a stamper carrier, which is in fact the movable mold half, in a conventional manner to form a stamper assembly. The apparatus removes one stamper assembly and replaces it with another to effect a change of the stamper. However, as this apparatus requires that each stamper be mounted to its own stamper carrier mold, this results in the stamper assemblies having much more mass than the stamper alone. To bring this increased mass up to mold operating temperature requires a significant time in which the mold can not be operated, necessitating that the apparatus preheat the stamper assembly before exchanging it for a stamper assembly already within the mold. Further, the requirement for a stamper carrier for each stamper increases the overall costs to mold the articles.